The overall goal of this proposal is to understand the role of CSF2 and its specific receptor CSF2R? in the development of t(8;21) leukemia. The 8 and 21 chromosome translocation, t(8;21)(q22;q22), is responsible for the development of 40% of FAB M2 type of acute myeloid leukemia (AML) and is reported in 8-15% of cases of AML, depending on geographic locations and ethnic groups. Age is the most correlated factor for AML. Most AML patients are over 60 years old at the initial diagnosis. However, t(8;21) AML patients are relatively young and most respond to initial chemotherapy well. Therefore, t(8;21) AML is generally considered with favorable prognosis. However, the cumulative incidence of relapse in 5 years is 47%, and the median overall survival time is only 5 years among de novo t(8;21) AML patients. Furthermore, therapy-related t(8;21) AML does not have a favorable outcome. Therefore, it is important to identify new approaches to eliminate t(8;21) leukemia stem cells during the initial induction treatment and to block the survival and proliferation of chemo-resistant leukemia cells. The fusion of ETO gene on chromosome 8 and AML1 gene on chromosome 21 in t(8;21) leads to the expression of the abnormal AML1-ETO protein. Using several transgenic mouse models and available human patient data, we discovered that colony stimulating factor 2 (CSF2) negatively regulates AML1-ETO induced self-renewal of hematopoietic stem/progenitor cells and AML development. Furthermore, expression of the CSF2-specific receptor subunit, CSF2R?, in t(8;21) AML cells reduces cell proliferation and survival. In this funding application, we will test the hypotheses that selected CSF2 downstream pathways play critical roles in suppression of t(8;21) leukemia and that CSF2R? may have functions independent of CSF2 signaling in inhibition of t(8;21) leukemia. We propose to perform the following studies to test the hypotheses: in specific aim 1, we will characterize the molecular pathways of CSF2 signaling that mediate inhibition of AML1-ETO induced leukemia; in specific aim 2, we will analyze the effect of CSF2RA expression on t(8;21) leukemogenesis. The proposed studies are based on our accumulated knowledge and recent novel findings involving CSF2 signaling and CSF2R? in AML1-ETO induced leukemia. Collectively, our proposal will address important unanswered questions in hematopoiesis and leukemogenesis, which aim to provide valuable insight into the treatment of t(8;21) leukemia.